The deposition or plating of zinc layers onto substrate surfaces finds widespread applications. Typically, zinc layers are very useful as functional coatings that are coated onto small iron-based parts, such as screws, nuts, and various structural parts. The zinc coating provides corrosion resistance, and in some cases, provides a smooth, decorative plating surface.
Moreover, due to its relatively low cost and good electrochemical properties, zinc has long been a desirable ingredient for the negative electrode or anode in rechargeable electrochemical cells (batteries) and fuel cells. A variety of energy storage devices that incorporate zinc have been produced, including zinc-air batteries, silver-zinc batteries, mercury-zinc batteries, and redox flow batteries. In the case of many of these devices, such as rechargeable, nickel-zinc flow batteries, the plating action of zinc is the primary electrochemical mechanism, somewhat analogous to the plating of articles for protective purposes. In other words, a zinc anode is alternately plated and then dissolved within an appropriate electrolyte bath. This type of redox reaction leads to the release of electrical energy, and alternately, the storage of electrical energy, when an external current is applied.
One problem with the use of zinc in electrochemical devices and plating systems is the formation of dendrites. The problem has been described in a number of references. The dendrites often form when zinc metal is deposited on an anode during the charging operation in an electrochemical cell, as zinc crystals form, typically as an overgrowth of a normal zinc deposit. The fractal zinc crystals or dendrites often have an elongate, pointed shape, and can penetrate the porous membrane of such a cell, causing the cell to short-circuit after only a small number of discharge and recharge cycles.
A number of methods have been disclosed for eliminating the problem of zinc dendrites, as described, for example, in U.S. Pat. No. 3,660,170 (Rampel). This patent describes the use of certain types of high-molecular weight polymers. These polymers can be used as additives in the electrolyte composition of an electrochemical cell, to inhibit the formation of the dendrites. Moreover, in other instances, extra layers of material are added to the membrane of an electrochemical cell, in a physical attempt to prevent penetration of the dendrites. However, additional techniques for addressing dendrite formation, especially during the operation of a cell at high current densities, would be welcome in the art. Preferred techniques would not require altering the membrane, or using specialized additives.
Additional needs are still present for zinc-based electrochemical cells. In the case of zinc plating baths, the ability to deposit smooth, adherent zinc layers by electroplating at high current densities would be of considerable interest, from both an economic viewpoint and a technical viewpoint. Moreover, in the case of zinc-based, rechargeable flow batteries, the ability to achieve greater energy and power (the amount of zinc stored per unit area; and high current density) would be very welcome in the art. This is especially the case for applications which are demanding greater performance from the battery, e.g., in grid applications, as well as electric vehicles.